The invention relates to a method for tuning dielectric antennas designed for operation especially in the microwave range. The invention also relates to an antenna structure and an apparatus in which the method is applied.
As portable apparatus comprising radio parts become more popular and smaller in size, also the antennas in them have to be small, located preferably within the covers of the apparatus. As frequencies higher than before are utilized, antennas naturally get smaller. For example, use of frequencies above the 2.4 GHz band is increasing. The size of the antenna structure can be further reduced through design. The structure may e.g. include planar elements and a dielectric medium. The smaller such an antenna, which deviates from the simple monopole, the more difficult it is to get its electrical characteristics within the limits specified. So, the drawback of a small antenna size is the difficulty of its fabrication.
The last phase in the manufacture of an antenna is the tuning of the antenna, i.e. making the resonance frequency or frequencies of the antenna exactly match the operating bands. The invention is directed to structures in which the radiating element of an antenna is a conductive layer on a surface of a dielectric board. In such antennas, the factor most contributing to the need of tuning is deviation in the thickness of the dielectric board. From the prior art a tuning method is known in which part of the radiating element is removed through mechanical working or by means of a laser beam. As the element size thus is reduced, the resonance frequency of the corresponding part of the antenna structure increases. Naturally the element originally has to be large enough so as to have a safe tuning margin. FIG. 1 depicts the aforementioned prior-art method and structure. There is a board-like dielectric block 110. On a first surface thereof, shown in the front, there is a radiating element 120 to a point F of which an antenna feed conductor is connected. On the opposing surface of the dielectric board there is a ground plane 130, or a conductive layer connected to the ground potential. The radiating element is short-circuited at a point S to the ground plane, which means the antenna is a planar inverted F antenna, or PIFA for short. In the example of FIG. 1 the radiating element 120 forms a thick II-shaped pattern on one end of which there are the aforementioned feed point and short-circuit point. The resonance frequency of the antenna is determined by the electrical length of the pattern. When tuning the antenna, part of the radiating element is removed from the end opposite to the feed point F, thereby decreasing the electrical length of the element. The figure shows an exemplary working border WB parallel to the end line of the element. Between the working border and the end of the element there is the conductive strip 121 to be removed.
A disadvantage of the method is that it is relatively inaccurate: Removing even a small amount of conductive material considerably changes the resonance frequency of the antenna. For example, in an antenna operating approximately at 2.5 GHz, the removal of a conductive strip one millimeter wide at the end of the element may change the resonance frequency for more than 100 MHz. Another disadvantage is that working the conductive layer may leave small conductive chips in the structure, risking a short-circuit as relatively strong electric fields occur in the antenna. If a laser beam is used in the working, an additional disadvantage is that a protection arrangement is required for the worker because when metal is removed by laser, plastic material is vaporized at the same time.
An object of the invention is to provide a novel and more advantageous method of tuning a dielectric antenna. A method according to the invention is characterized by that which is expressed in the independent claim 1. An antenna structure according to the invention is characterized in that which is expressed in the independent claim 5. An apparatus according to the invention is characterized in that which is expressed in the independent claim 10. Advantageous embodiments of the invention are disclosed in the other claims.
The basic idea of the invention is as follows: An antenna is tuned by removing material from a dielectric block placed between conductive elements. The removal of dielectric material decreases the average dielectric constant in the space between the conductive planes, resulting in an increase in the resonance frequency of the antenna. The antenna is advantageously fabricated such that the conductive elements on the opposing surfaces of the dielectric block are shaped identical and are located symmetrically with respect to each other so that the tuning of the antenna will not affect the other electrical characteristics of the antenna but the resonance frequency only.
An advantage of the invention is that the method according to the invention enables accurate tuning of an antenna since removing a small amount of material from the dielectric medium changes the resonance frequency of the antenna only relatively little. Another advantage of the invention is that with the method according to the invention, structural defects in the dielectric medium will be automatically compensated for. A further advantage of the invention is that the working of the dielectric material will never produce additional small conductive formations in the antenna structure. A further advantage of the invention is that plastics which usually are used as dielectric material are easy to work. A further advantage of the invention is that the mechanical working of the plastic will not require protection of the worker. A further advantage of the invention is that the antenna is easy to tune even in the finished product, because tuning only requires an access to one side of the antenna. A further advantage of the invention is that with the structure according to it the tuning of the antenna will not affect other electrical characteristics than the resonance frequency.